Hypertension is a cardiovascular risk factor that often leads to target organ damage. Angiotensin-converting enzyme inhibitors (ACEi) significantly reduce cardiovascular events, especially in high-risk patients. The effects of ACEi are mediated by inhibition of both the conversion of Ang I to Ang II and kinin degradation. We have evidence that in hypertension another peptide hydrolyzed by ACE, N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP), prevents and reverses cardiac fibrosis without altering blood pressure (BP) or cardiocyte hypertrophy. This is the first demonstration that administration of Ac-SDKP has an effect on cardiac fibrosis; however, we do not know whether Ac-SDKP has a physiological role, the mechanism by which it inhibits fibrosis or whether it contributes to the cardiovascular protective effects of ACEi. In this project we propose to test the general hypothesis that in hypertension cardiac fibrosis is the result of an alteration of the balance between pro-fibrotic and pro-inflammatory vs anti-fibrotic and anti-inflammatory systems. Ac-SDKP alters this balance in favor of the latter, reversing fibrosis (an important component of target organ damage) and improving cardiac function. The mechanisms by which Ac-SDKP antagonizes pro-fibrotic stimuli are: a) directly by inhibiting fibroblast proliferation and collagen synthesis and b) indirectly by acting as an anti-inflammatory cytokine, thus inhibiting production of TGF beta 1 and other cytokines and macrophage activation and infiltration. We also hypothesize that part of the anti-fibrotic effect of ACEi on target organ damage is mediated by Ac-SDKP interacting synergistically with kinins and NO. To test this hypothesis, we propose to conduct in vivo studies, using a combination of physiological, pharmacological and molecular approaches (gene deletion). In the first two aims we will determine the mechanisms by which Ac-SDKP prevents and reverses cardiac fibrosis. In Aim 1 we will study its effect on fibroblast proliferation and collagen synthesis and degradation. In Aim 2 we will study whether Ac-SDKP inhibits cardiac fibrosis in part by acting as an anti-inflammatory cytokine, decreasing proinflammatory cytokines and macrophage activation and infiltration and reactive oxygen species production. In Aim 3, we will study whether in hypertension Ac- SDKP improves diastolic dysfunction by reversing cardiac fibrosis. In Aim 4 we will determine whether endogenous Ac-SDKP antagonizes the inflammatory and fibrotic effect of angiotensin II (Ang II), aldosterone, and myocardial infarction (MI). In addition, we will study whether part of the cardiovascular protective effect of ACEi is due to an increase in Ac-SDKP, which interacts with kinins and NO to decrease extracellular matrix deposition in the cardiovascular system. These studies are significant since they will demonstrate: 1) the mechanism by which Ac-SDKP decreases cardiac fibrosis; 2) whether it has a therapeutic effect, improving diastolic and systolic dysfunction by reversing cardiac fibrosis; 3) whether it has a physiological role by antagonizing pro-fibrotic stimuli in the cardiovascular system; and 4) whether it mediates the cardiovascular protective effect of ACEi. In the future, non-peptidic analogues of Ac-SDKP could be developed to treat fibrosis in hypertension, aging, heart failure (HF) post-MI, diabetes and other diseases.